I don't think anyone but Canon can answer your question with any sort of reliability, so take all of this with a grain of salt.
The impression I've been given from all the rumours is that Canon wouldn't release a 7D III along side a crop sensor RF mount equivalent because they may not release release a 7D III at all, let alone a 7D III with a crop equivalent. I believe the 7D series had been updated on a 5 year cycle, and we're right at year 5 of waiting for a 7D III, so the feeling that no 7D III is coming may be realistic. Why they'd choose not to continue the 7D line is anyone's guess, but if they stopped producing a camera which has strong sales then I'd bet they have a reason which makes sense for them. Here's a few grasping at straws ideas on why they may not refresh the 7D:
- Maybe it was cannibalizing higher end bodies for the price point so they don't plan on replacing it? Maybe that's why the 90D got a bit of a specs boost from the 80D?
- Maybe they'll release a new camera in that niche on EF-M mount? I.e. the m5II?
- Maybe they'll release an RF mount body with a new and improved autofocus system to fill that niche instead of the 7D? Mirrorless bodies could hold a lot of promise for a 7D - style camera with really great autofocus tracking, and incredibly fast burst rates - the m6II is a good example of what kind of performance is possible in a crop mirrorless body.
- Maybe the 7D series benefit - a relatively cheap and ruggedized body with great autofocus and burst rates - will not stand out with some of the technical advancement of late? Crazy fast burst rates are moving into lower tier camera bodies (i.e. M6II), incredible autofocus capabilities seem to be more software based than hardware (on mirrorless anyway - though even lower-end mirrorless bodies are getting very advanced autofocus capabilities), and the only thing missing seems to be the ruggedized body. Maybe that's not enough niche for Canon to justify? Rumour has it Nikon went the same route and will not update the D500 (the 7DII's most direct competitor), so maybe the niche for rugged/price-conscious/high-burst rate/great autofocus just doesn't make sense from a business standpoint any longer.
Really, it's anyone's guess. The only fact we have to work from here is that a 7D III is late from a release cycle point of view, and whether it is coming at all remains to be seen. Probably not an answer which will make anyone happy, but the fact is no-one (myself included) knows why the 7D III isn't here yet or what direction Canon is headed with the 7D series.